<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        body{
            margin: 20px 40px 20px 40px;
            flex: content-box;
        }
        .user{
            align-content: center;
        }
    </style>
</head>
<body>
    <form method="post" class="user">
        <label for="username">Username</label><br>
        <input type="text" name="username" placeholder="Héctor Rivera"><br>
        <label for="password">Password</label><br>
        <input type="password" name="password" placeholder="19001130"><br>
        <input id="remember" name="remember" type="checkbox" checked>
        <label for="remember"><small>Remember me</small></label><br>
        <input type="submit" name="submit" value="Log in">
    </form>
</body>
</html>